Motor vehicles today, e.g. cars, trucks and buses, are usually provided with cruise control. An object of cruise control is to achieve a uniform predetermined vehicle speed. This is done either by adjusting the engine torque to avoid retardation, or by applying brake action on downhill runs where the vehicle is accelerated by its own weight. A more general object of cruise control is to achieve convenient driving and greater comfort for the vehicle's driver.
A driver of a motor vehicle with cruise control usually chooses a set speed vset as the speed which he/she wishes the vehicle to maintain as long as possible on level roads, depending for example on engine performance and vehicle weight or on whether there is another vehicle moving at low speed in front. A cruise control then provides an engine of the vehicle with a reference speed vref which is used for controlling the engine. The set speed vset may therefore be regarded as an input signal to the cruise control, and the reference speed vref as an output signal from the cruise control, which is used for controlling the engine.
In today's traditional cruise controls (CC), the reference speed vref is identical with the set speed vset set by the user of the system, e.g. a driver of the vehicle. Today's traditional cruise controls therefore maintain a constant reference speed corresponding to the set speed vset set by the driver. The value of the reference speed vref is here altered only when the user him/herself adjusts it during the journey.
There are today cruise controls, so-called reference-speed-regulating cruise controls, e.g. Ecocruise cruise control, which try to estimate current running resistance and also have knowledge of historical running resistance.
An experienced driver using a vehicle without cruise control can reduce fuel consumption by adapting his/her driving to the characteristics of the road ahead so that unnecessary braking and/or fuel-consuming acceleration can be avoided. A further development of these reference-speed-regulating cruise controls tries to mimic the experienced driver's adaptive driving on the basis of knowledge of the road ahead, so that fuel consumption can be kept as low as possible, since this very greatly affects profitability for an owner of the vehicle, e.g. a haulage company or the like.
An example of such a further development of a reference-speed-regulating cruise control is a “look ahead” cruise control (LACC), i.e. a strategic cruise control which uses knowledge of sections of road ahead, i.e. knowledge of the nature of the road in front, to determine the configuration of the reference speed vref. Here the reference speed vref is therefore allowed, within a speed range, to differ from the set speed vset chosen by the driver, in order to drive in a way which saves more fuel but is also more comfortable and safer.
Knowledge of the road section ahead may for example comprise prevailing topography, road curvature, traffic situation, roadworks, traffic density and state of the road. It may also comprise a speed limit for the road section ahead and a traffic sign beside the road. This knowledge may for example be obtained from location information, e.g. GPS (global positioning system) information, map information and/or topographical map information, weather reports, information communicated between different vehicles and information communicated by radio. These different types of knowledge may be used in various ways. For example, knowledge of a speed limit ahead may be used for fuel efficiency by lowering the vehicle's speed before the speed limit zone is reached. Similarly, knowledge of a road sign conveying information about, for example, a roundabout or intersection ahead may also be used for fuel efficiency by braking before reaching the roundabout or intersection.
A reference-speed-regulating cruise control may, for example, predict the vehicle's speed along a horizon of any suitable length, e.g. about 1-2 km. The vehicle's future speed along the horizon is predicted in various ways such as driving with traditional cruise control at a reference speed vref which is the same as the set speed vset, or varying the reference speed vref relative to the set speed vset.
A look-ahead cruise control (LACC), for example, allows the reference speed vref to be raised, before a steep climb, to a level above the set speed uset, since the vehicle will presumably lose speed on the steep upgrade owing to high train weight relative to engine performance. Similarly, the LACC allows the reference speed vref to drop to a level below the set-speed vset before a steep downgrade, since on such a downhill run the vehicle will presumably be accelerated by its high train weight. The concept here is that it is better from a fuel economy perspective to take advantage of the vehicle's acceleration by its own weight downhill than to initially accelerate before the downgrade and then brake downhill. The LACC can thus reduce fuel consumption without journey time being affected, while at the same time achieving comfortable and safe driving.
There are also cruise controls which use a current running resistance as a basis for deciding how the vehicle's speed should vary. This means that the reference speed vref in these cruise controls can be allowed to deviate from the set speed vset on the basis of at least one characteristic of the running resistance, e.g. its magnitude and/or pattern over time.